Flat pipe comprising a return bend section and a heat exchanger constructed therewith

ABSTRACT

The invention related to a flat tube ( 1 ) comprising a return bend section ( 3 ), inside of which the flat tube ( 1 ) is bent back in such a manner that both planar tube sections ( 2   a,    2   b ) thereof, which are connect to the return bend section, extend in a longitudinal direction with opposite flow-through directions ( 4   a,    4   b ) and with longitudinal axes ( 5   a,    5   b ) that are offset with regard to one another at least in the transverse direction (y). The invention provides that the return bend section ( 3 ) is formed in such a manner that a main bending axis (A) extends parallel to the flat tube to be plane and at a predeterminable angle to the tube longitudinal extension, whereby the flat tube plane is determined by the longitudinal and width extension of the flat tube ( 1 ).

The invention relates to a flat tube according to the preamble of claim 1 and to a heat exchanger constructed therewith.

A generic flat tube comprising a return bend section and a heat exchanger comprising a tube block constructed from this type of flat tube are described in the laid-open publication DE 198 30 863 A1. To produce, there, the flat tube comprising a return bend section, the flat tube is bent around in such a way that its two planar tube sections adjoining it run in a longitudinal direction with opposite throughflow directions and with longitudinal axes offset relative to one another at least in the transverse direction.

The laid-open publication EP 0 659 500 A1 likewise describes a flat tube comprising a return bend section and a heat exchanger comprising a tube block constructed from this type of flat tube. To produce the flat tube there, a rectilinear flat tube blank is first bent out of the flat tube plane in a U-shaped manner, until the flat tube legs run parallel to one another, after which the latter are twisted in each case at 90° with respect to the U-bend region. The flat tube thereby occurring thus possesses two planar tube sections which lie in one plane and the issue ends of which lie on the same side opposite to the return bend section. The angle which the flat tube transverse axis forms along the return bend section with the plane in which the rectilinear tube legs lie first increases, over one twist region, from zero to the value of 90° present at the head end of the return bend section, in order then to decrease to 0° again over the other twist region. What is considered to be a disadvantage in the return bend section described is that the extent of the flat tube perpendicular to the plane of the planar tube legs in the head region of the return bend section always corresponds to a flat tube width and therefore cannot be reduced as required, so that the dimensions of the associated heat exchanger tube block cannot be influenced in the direction perpendicular to the plane of the rectilinear flat tube legs.

The object of the invention is to provide a flat tube comprising a return bend section, which flat tube can be produced relatively simply and is suitable for the construction of highly pressure-resistant heat exchangers having a small overall space, and to specify a heat exchanger constructed from such flat tubes.

According to the invention, this object is achieved, with regard to the flat tube, by means of the features of patent claim 1 and, with regard to a heat exchanger, by means of the features of patent claims 13, 17 or 18.

The dependent patent claims relate to advantageous refinements and developments of the invention.

The main idea of the invention is to design a return bend section in such a way that a main bend axis runs parallel to the flat tube plane and at a predeterminable angle with respect to the tube longitudinal extent, the flat tube plane being determined by the extent of the flat tube in terms of length and of width. In an advantageous embodiment, the predeterminable angle amounts to 90°, that is to say the main bend axis then runs perpendicularly with respect to the tube longitudinal extent.

The flat tube according to the invention, during the forming operation, is displaced by the amount of a distance s, parallel to the tube extent, in the flat tube plane in the region of the return bend section, the distance s being composed of a flat tube width b and of a desired spacing d between the flat tube sections after the forming operation.

In the flat tube according to the invention, an angle α with which the flat tube sections merge into the return bend section can be selected freely during the forming of the flat tubes and, in an advantageous embodiment of the invention, lies in the range of 13°<α<67°.

In an advantageous embodiment of the flat tube according to the invention, the angle α and/or the distance s are/is achieved by means of at least one bending operation about at least one bend axis (B) which runs perpendicularly with respect to the flat tube plane.

In a particularly advantageous embodiment of the flat tube according to the invention, the displacement of the flat tube is achieved by means of two bending operations about two bend axes which are carried out before or after the main bending operation about the first bend axis, the first bend axis running in the middle of the offset region, the offset region being approximately twice as long as the return bend section. This applies particularly when a main bending operation is carried out about a main bend axis which runs perpendicularly with respect to the tube extent.

In the hitherto described flat tube according to the invention, after the forming operation the two planar tube sections adjoining the return bend section are arranged so as to lie perpendicularly with respect to the stack direction z in parallel planes offset laterally in relation to one another, preferably with the spacing d in the transverse direction y of between 0.2 mm and 20 mm. If flat tubes bent around once in this way are used, when the direction of the offset is changed at each deflection a tube block in a serpentine type of construction, in which the serpentines run so as to be offset laterally, can be formed. The tube block thus formed has a depth of double the flat tube width plus said spacing d between the planar tube sections. With flat tubes bent around more than once, offset, in the same direction, the tube block depth per return bend section increases by the amount of the flat tube width plus said transverse spacing d between the planar tube sections. As a result of the transverse spacing, corresponding gaps between the flat tube sections are formed in a tube block constructed by means of such flat tubes, thus making it easier for condensation water to be separated, for example when the tube block is employed in an evaporator of a motor vehicle air conditioning system.

In order to ensure that the flat tubes lie in a common plane, in a further forming step the return bend section is formed in such a way that the two tube sections lie next to one another and parallel, at the spacing d, in a common plane. This may take place by means of a symmetrical or asymmetric forming of the return bend section.

As a result of the change between the return bend sections, in which the flat tube sections lie in the same plane, designated below as first return bend sections, and the return bend sections, in which the flat tubes lie in different planes, designated below as second return bend sections, a tube block in a serpentine type of construction can be implemented, the depth of which is dependent on the number of first return bend sections formed one behind the other. As a result of a constant change of first and second return bend sections in which the offset is likewise formed in the opposite direction, for example, a tube block in a serpentine type of construction, with a depth of double the flat tube width plus said spacing d between the planar tube sections, can be implemented, in which a thermal control medium, for example a refrigerant or a coolant, first flows through the flat tube sections which lie in a common plane and then flows through the flat tube sections which lie in the next common plane in the stack direction or opposite to the stack direction.

However, it is, moreover, also possible to achieve a serpentine form of construction in that a number of second return bend sections are designed without a lateral offset, designated below as third return bend sections, for example in the stack direction, and in that, subsequently, a first return bend section is formed which is followed by a number of second return bend sections. A second return bend section may, of course, also be arranged instead of the first return bend section. In such a tube block, first, the thermal control medium flows through all the flat tube sections which lie one above the other in a front region, that is to say in a region facing the air, and, subsequently, after a first or a second return bend section, flows through all the flat tube sections lying in a rear region, and the order of the throughflow may also be opposite, that is to say the thermal control medium flows first through the rear region and then through the front region, and, depending on the application, the throughflow may take place from the top downward or from the bottom upward.

In an alternative procedure for the configuration of the return bend section, the main bending operation about the main bend axis is carried out at a predeterminable angle with respect to the tube longitudinal extent, the predeterminable angle corresponding essentially to the angle α with which the flat tube sections merge in to the return bend section.

After the main bending operation, the two flat tube sections lie in two planes parallel to one another, the two flat tube sections forming an angle having a value of 2a. In order to obtain parallel tube legs, the two tube legs are in each case formed, by means of a further bending operation about a bend axis running perpendicularly with respect to the flat tube plane, in such a way that they merge into the return bend section in each case at the angle α. The procedure described gives rise in another way to the required flat tube offset already described.

The further forming steps are carried out in a similar way to those already described, in order to ensure that the two flat tube sections lie next to one another and parallel, at the spacing d, in a common plane. As already stated, this may take place by means of a symmetrical or asymmetric forming of the return bend section.

In principle, however, it is also possible to reverse the order of the forming steps and first form the two tube sections by means of a symmetrical or asymmetric forming of the return bend section, in such a way that they lie in a common plane and form the angle 2α, and subsequently to carry out the two bending operations described above, in order to ensure that the two tube sections lie parallel and next to one another, at their spacing d, in the common plane.

Overall, what is achieved by means of the configuration according to the invention of the return bend section is that its extent in the stack direction can be kept markedly smaller than the flat tube width. Accordingly, the interspaces between adjacent flat tubes in the stack-shaped construction of a tube block from these flat tubes do not need to be as large as or are kept no larger than the flat tube width, but, instead, may be markedly narrower, this being conducive to the production of a compact and pressure-resistant heat exchanger. Moreover, the return bend section can be implemented by means of relatively simple tube bending operations. The flat tube may in this case be bent around in this way once or more than once, its depth extent, that is to say its extent in the transverse direction, as defined above, increasing with each bend when the lateral offset always takes place in the same direction. As a result, it is possible, by means of relatively narrow pressure-resistant flat tubes, to form a tube block of any desired depth, that is to say extending in the transverse direction, this transverse or depth direction normally being that direction in which a medium to be cooled or to be heated is conducted through the heat exchanger past the flat tube surfaces on the outside. In this case, mostly, additional heat conduction means between the tube block sections succeeding one another in the stack direction are provided in order to improve the heat transmission. Since, as stated, the tube interspaces can be kept very narrow, correspondingly low heat-conducting corrugated ribs can also be used, thus likewise improving the compactness and stability of a tube/rib block thus formed.

To produce a flat tube heat exchanger for motor air conditioning systems, a plurality of flat tubes according to the invention are stacked one above the other in a stack direction z in order to form a tube block. The flat tubes issue in each case at one end into at least one collecting duct arranged laterally and running in the stack direction of the tube block, and at least one of the two tube sections connected to one another via the return bend section may form a tube serpentine coiled in the stack direction z, while the two flat tube ends lie on the same or on opposite sides, and at least one of the two tube ends may be twisted at an angle of between 0° and 90°.

By the flat tubes being designed according to the invention with a 180° deflection in the flow direction, it is possible to implement a smaller overall space for the heat exchangers, such as, for example, a gas cooler or an evaporator, since narrower spacings in the stack direction and/or between the tubes can be implemented. Moreover, a springing open of the flat tube legs is virtually avoided. A further advantage is that the heat exchangers constructed with the flat tubes according to the invention have a more rigid design with narrower tolerances.

In the present gas cooler variant, the refrigerant is routed in a flat tube in cross countercurrent to the air. At the opposite block end, a deflection through 180° takes place, that is to say the flat tube runs back in the same plane as on the outward path, but so as to be offset laterally by the amount of distance s, so that the outgoing section of the flat tube is distanced from the returning section by the amount of a spacing d. The two flat tube sections lie in the same plane which is defined by the extent of the flat tubes in terms of length and of width in their straight sections. Forming is carried out preferably in three steps. In the first step, the flat tube undergoes a lateral offset from the stretched state. The amount of the offset s corresponds to the sum of the flat tube width b and spacing d. Subsequently, bending takes place with a radius r about a main bending axis A parallel to the flat tube plane and perpendicularly with respect to the tube extent, r being the inner radius of the bend. The main bend axis A lies approximately in the middle of the offset region. The sections of the flat tube subsequently lie parallel to one another in different planes. In a third step, the return bend section is formed in such a way that the flat tube sections lie in a common plane again. The formed return bend section may either lie completely below or above with respect to the common flat tube plane or lie symmetrically with respect to this common plane. Moreover, any desired asymmetric positions of the return bend section in relation to the common plane are possible. Alternatively to the forming order described, the forming steps may also be interchanged.

For the offset of the flat tube in the plane, the following geometric relations can be set up: the angle α at which the flat tube runs in the offset region, contrary to the original tube extent, is obtained from α=arctan (b+d/U). With b: flat tube width, d: distance between the flat tubes, U: offset region.

For the offset region U, the following estimation: U=2Πr is obtained, r being the inner radius of the 180° bend. The following applies to the maximum inner radius r: (h_(r)−d_(FR))/2, h_(r) being a rib height and d_(FR) being a flat tube thickness. The flat tube thickness d_(FR) seems to be an appropriate lower limit value for r min. According to these formulae, an appropriate value for α lies within the limits 13°<α 67°.

In an advantageous embodiment, the flat tube according to the invention forms a serpentine flat tube, in that at least one of the two flat tube sections connected via a return bend section is bent in a stack direction to form a tube serpentine, that is to say consists of third return bend sections succeeding one another in the stack direction and having the corresponding planar tube sections. By means of flat tubes thus configured, a serpentine heat exchanger, as it is known, can be constructed with any desired number of serpentine block parts succeeding one another in the depth direction.

In a further embodiment of the flat tube according to the invention, the issue ends lie on the same or on opposite sides, at least one end, preferably both ends, being twisted with respect to the adjacent middle region. As a result of this twisting, the flat tube transverse axis is rotated in the direction of the issue end toward the stack direction, so that the extent of the flat tube ends in the transverse direction can be kept smaller than the flat tube width. Twisting takes place at most through 90°, so that then, with the planar tube sections running perpendicularly with respect to the stack direction, the tube ends lie parallel to the stack direction and their extent in the transverse direction is only as large as the flat tube thickness. This makes it possible to have an arrangement, comparatively narrow in the depth direction of a tube block constructed therewith, of associated collecting and distributing ducts extending on the respective tube block sides in the stack direction.

A heat exchanger is characterized by the use of one or more of the flat tubes according to the invention in the construction of a corresponding tube block, with the abovementioned properties and advantages of such a tube block construction. In particular, in this way, a compact and highly pressure-resistant evaporator with relatively low weight, a small inner volume and good condensation water separation can be implemented for an air conditioning system of a motor vehicle, preferably multichamber flat tubes being employed. The heat exchanger can be implemented both in a single-layer type of construction, in which the flat tube sections between two return bend sections or between a return bend section and a flat tube end consist of a planar rectilinear tube section, or in a serpentine type of construction, in which these flat tube sections are bent to form a tube coil.

In a developed heat exchanger, the tube ends of the flat tubes used and consequently also the associated collecting and distributing ducts, designated uniformly below as collecting ducts for the sake of simplicity, are located on opposite tube block sides. The connecting ducts may then be formed in each case by a header box or header tube which run on the respective tube block side along the stack direction, also designated as the vertical block direction, and which serve for supplying or discharging the thermal control medium conducted through the tube interior to or from the individual flat tubes.

In an alternative development of the invention, the flat tubes all issue on the same tube block side. Owing to the configuration of the flat tubes, the two tube ends of each flat tube are in this case offset in relation to one another in the block depth direction, so that they can be assigned two collecting ducts lying correspondingly next to one another in the block depth direction. Accordingly, the supply and discharge of the thermal control medium conducted through the tube interior take place on the same heat exchanger side.

In a further refinement of this heat exchanger type with two collecting ducts lying next to one another on the same tube block side, there is provision for these collecting ducts to be formed by two separate header tubes or header boxes, designated uniformly below as header tubes for the sake of simplicity, or by one common header tube. The latter can be implemented in that an initially uniform header tube interior is divided off by means of a longitudinal partition into the two collecting ducts, or in that the header tube is manufactured as an extruded tube profile with two separate hollow chambers forming the collecting ducts.

In a developed heat exchanger, at least one of the two header tubes or at least one of the two hollow chambers of a longitudinally divided header tube is subdivided by means of transverse partitions into a plurality of collecting ducts separated from one another in the vertical block direction. As a result, a grouped serial throughflow of the flat tubes in the tube block is achieved, in that the thermal control medium supplied to the tube block via a first collecting duct of the transversely divided header tube or of the transversely divided hollow chamber is first fed only into the part of all the flat tubes which issues there. The collecting ducts into which this part of the flat tubes issues with the other tube end then functions as a deflecting duct, in which the thermal control medium is deflected from the flat tubes issuing there into a further part of all the flat tubes which likewise issues there with one end. The number and position of the transverse partitions determine the division of the flat tubes into successive-throughflow groups of parallel-throughflow flat tubes.

In a flat tube produced according to the invention, the arrangement of the flat tubes with regard to an air stream remains unchanged despite the return bend section, that is to say a flat tube side facing the air continues to face the air even after the return bend section and a flat tube side facing away from the air continues to face away from the air even after the return bend section.

In contrast to this, a position of the tube underside or tube top side is changed by means of the return bend section, that is to say the tube underside of the flat tube becomes a tube top side of the flat tube and a tube top side of the flat tube becomes the tube underside of the flat tube.

Advantageous embodiments of the invention are illustrated in the drawings and are described below. In the drawings:

FIG. 1 shows a top view of a flat tube with a return bend section and with twisted tube ends,

FIG. 2 a shows a side view, along the arrow I in FIG. 1, of a flat tube with a secondary return bend section;

FIGS. 2 b to 2 d shows side views, along the arrow I of FIG. 1, of flat tubes with differently designed first return bend sections;

FIG. 3 a shows a top view of a flat tube before a bending operation about a main bend axis A;

FIG. 3 b shows a top view of a flat tube after a bending operation about a main bend axis A;

FIG. 4 shows a side view of a detail of a tube/rib block of a heat exchanger, said tube/rib block being constructed from flat tubes according to FIGS. 1 and 2;

FIG. 5 shows a side view of a detail of a tube/rib block of a heat exchanger with serpentine-shaped flat tubes.

The flat tube 1 shown in the top view in FIG. 1 is manufactured in one piece from a rectilinear multi chamber profile, using suitable bending operations. It contains two planar rectilinear tube sections 2 a, 2 b which are connected to one another via a return bend section 3 and which have opposite throughflow directions for a thermal control medium, for example a refrigerant of a motor vehicle air conditioning system, conducted through the plurality of parallel chambers inside the flat tube 1. One of the two possible flow profiles is illustrated in FIG. 1 by corresponding flow arrows 4 a, 4 b. The longitudinal axes 5 a, 5 b, running parallel to the throughflow directions 4 a, 4 b, of the two planar rectilinear tube sections 2 a, 2 b define a longitudinal direction x and are offset relative to one another in a transverse direction y perpendicular thereto. As is evident particularly from the side views of FIG. 2 b to 2 c, the two planar tube sections 2 a, 2 b lie with a first return bend section 3 in a common x-y plane perpendicular to a stack direction z in which a plurality of such flat tubes are stacked one on the other to form a heat exchanger tube block, as explained in more detail below with reference to FIG. 4 and 5. For greater clarity, in each case the corresponding coordinate axes x, y, z are depicted in FIG. 1 to 5. The return bend section 3 is obtained in that the initial rectilinear flat tube profile of a desired width b is displaced by the amount of a distance s, parallel to the tube extent, in the flat tube plane in the region of an offset region U, as illustrated in FIG. 3 a, said distance being composed of the tube width b and of the desired spacing d. The displacement or offset may in this case take place in a positive y-direction or, opposite, in a negative y-direction. The transition between the flat tube sections 2 a, 2 b and return bend section 3 takes place at a predeterminable angle α. The angle α and/or the distance s are/is in this case achieved by means of at least one bending operation about at least one bend axis B1, B2 running perpendicularly with respect to the flat tube plane. Preferably, the described offset by the amount of the distance s is achieved by means of two bending operations about the bend axes B1 and B2 illustrated in FIG. 3 a, these two bending operations preferably being carried out before the bending operation about the main bend axis A. In the exemplary embodiment illustrated, the main bend axis A runs in the middle of the offset region U, the offset region U being approximately twice as long as the return bend section 3.

The two rectilinear tube sections 2 a, 2 b of the flat tube 1 are obtained in a way described. After the offset of the flat tube 1 and the main bending operation, the two rectilinear tube sections 2 a, 2 b lie, as illustrated in FIG. 2 a, offset in planes parallel to one another, with a selectable spacing 2 r in the z-direction and the selectable spacing d in the y-direction, the following applying to the maximum inner radius r: (h_(r)−d_(FR))/2, h_(r) being the rib height and d_(FR) being the flat tube thickness, thus resulting in the flat tube thickness d_(FR) as an appropriate lower limit value for r. According to these formulae, an appropriate value for the angle α lies within the limits 13°≦α≦67°. The selectable spacing is preferably between about 0.2 mm and 20 mm, while the flat tube width b is typically between one and a few centimeters.

While the rectilinear tube sections 2 a, 2 b are connected to one another on one side via the return bend section 3, they both run out on the opposite side in the form of twisted tube ends 6 a, 6 b. Twisting takes place about the respective longitudinal midaxis 5 a, 5 b, alternatively also about a longitudinal axis parallel thereto, that is to say with a transverse offset with respect to the longitudinal mid axis of any desired angle between 0° and 90°, the twist angle being approximately 90° in the instance shown.

It becomes clear from FIG. 2 that, because of the depicted formation of the return bend section 3, the height c of the return bend section 3 and consequently the extent in the stack direction z are small and can be selected as a function of the bend radius. In particular, this height c of the return bend section 3 remains markedly smaller than the flat tube width c. As a result, a plurality of such flat tubes can be layered one above the other in a heat exchanger tube block with a stack height which can be kept markedly smaller than the flat tube width, as the heat exchanger examples described below show. A further modification of the flat tube of FIG. 1 and 2 may be that, as shown in FIG. 2 a, the two planar tube sections 2 a, 2 b lie in two x-y planes offset in relation to one another. In this case, the transverse direction y is defined in that it is perpendicular both to the longitudinal direction x of the rectilinear tube sections and to the tube block stack direction z.

FIG. 3 b shows an alternative possibility for the configuration of a return bend section 3 after the main bending operation. As is evident from FIG. 3 b, here, first the main bending operation about the bend axis A is carried out, before the offset is implemented by means of further bending operations about a bend axis B3. The main bend axis A in this case runs at the predeterminable angle α within the limits 13°≦α≦67° with respect to the tube longitudinal extent. After the main bending operation, the two tube sections are in each case bent inward about the bend axis 3 according to the arrows. According to the illustration in FIG. 3 b, the spacing d between the flat tubes is implemented by means of a boundary, in the example illustrated is implemented by means of a boundary strip having the width d, in the example illustrated the bend axis B3 being implemented by means of an upper end of the boundary strip. The flat tube sections 2 a and 2 b illustrated lie in different parallel planes and form an angle of 2 a. After the additional bending operations, the two flat tube sections 2 a and 2 b lie parallel to one another in the different parallel planes, as illustrated in FIG. 2 a, so that the further forming steps already described can be carried out, in order to ensure that the two flat tube sections 2 a, 2 b lie parallel, with the spacing d, in a common plane (see FIG. 2 b to 2 c).

FIG. 4 and 5 show an application for the flat tube type of FIG. 1 and 2 in the form of a tube/rib block 9 of an evaporator 10, such as can be used, in particular, in motor vehicle air conditioning systems. It goes without saying that the heat exchanger, a detail of which is shown, can also be used, depending on its design, for any other desired heat transmission purposes, for example as a gas cooler. As is evident from FIG. 4, this evaporator 10 contains, between two end cover plates 11, 12, a stack of a plurality of flat tubes 1 according to FIG. 1 and 2 with intermediate heat-conductive corrugated ribs 8. The height of the heat-conducting ribs 8 corresponds approximately to the height c of the flat tube return bend sections 3 and is consequently markedly smaller than the flat tube width b.

Using the flat tube 1 of FIG. 1 and 2, a tube/rib block 9 with a structure which is two-part in depth, that is to say in the y-direction, is formed, in each of the two block parts in each case the tube sections with the same throughflow direction lying one above the other in the stack direction z. Between the two block parts is formed a gap corresponding to the spacing d of the two rectilinear tube sections 2 a, 2 b of each flat tube 1. In the exemplary embodiment illustrated, the corrugated ribs 8 extend in one part over the entire flat tube depth and consequently also over this gap, and can, if required, project on both sides, that is to say on the front side and on the rear side of the block. It is also possible, however, to use multipart, in particular two-part corrugated ribs 8. The block front side is in this case defined in that it faces the flow of a second thermal control medium, for example supply air to be cooled for a vehicle interior, conducted over the evaporator surfaces on the outside, in the tube transverse direction y, that is to say in the block depth direction.

The transverse extent of the flat tube issue ends is smaller than the flat tube width b on account of their twisting. This makes it easier to connect two associated collecting ducts, not shown in FIG. 4, which may be formed in each case by a header box or header tube, of which the transverse extent in the y-direction does not need to be any greater than the flat tube width b and, in the case of a twist angle of the flat tube ends of approximately 90°, even needs in its diameter to be only slightly greater than the flat tube thickness. It is therefore easily possible to arrange two header tubes so as to run next to one another in the stack direction z in the respective tube block side, in order in each case to receive one of the two ends of each flat tube 1. Alternatively, a common header tube may be provided for both stack rows of tube ends 6 a, 6 b, said common header tube being subdivided into the two required separate collecting ducts by means of a longitudinal partition.

It is shown that the evaporator 10 can be implemented in a compact form of construction and in a highly pressure-resistant way by means of the tube/rib block 9 thus formed and at the same time has a high heat transmission efficiency. By the flat tubes being bent around to form two tube sections 2 a, 2 b offset in the block depth, it is possible, by means of relatively narrow flat tubes, to achieve a heat transmission capacity for which nonbent flat tubes having approximately double the width would otherwise be required. What is achieved at the same time by means of the once-only flat tube deflection is that the thermal control medium routed through the tube interior can be supplied and discharged from one and the same tube block side, this being advantageous in many applications.

FIG. 5 shows an exemplary embodiment in a serpentine type of construction. The view of a detail in FIG. 5 shows in this case a plurality of serpentine flat tubes 13 which are stacked one above the other in any desired number to form the serpentine tube block there. The serpentine flat tube 13 used for this purpose is largely structurally identical to that of FIG. 1 and 2, with the exception that each of the two sides of the return bend section 3 identical to that of FIG. 1 and 2 has adjoining it not simply a rectilinear single-layer tube section, but a tube coil section 12 which is multiply coiled in a serpentine-shaped manner the latter thus again being located opposite one another, offset by the amount of a corresponding gap in the block depth direction. As is customary, the serpentine windings 12 of the respective tube coil section 13 are formed by the flat tube being bent at the respective point around the tube transverse axis there at an angle of 180°. Between the individual tube coil windings 13 and between successive serpentine flat tubes 12, heat-conductive corrugated ribs 8 are introduced, with optional projection, continuously from the block front side as far as the block rear side. It goes without saying that, here, as also in the example of FIG. 4 and 5, a corrugated rib row may in each case be provided, instead, for each of the two tube block rows offset in the block depth direction, in which case the gap between the two block rows may also remain free. Instead of this division by half with two corrugated ribs of identical width, of course, any other desired number of corrugated ribs and/or corrugated ribs of different depth may be employed over the tube block depth in each corrugated rib layer, for example first corrugated rib extending over two thirds of the tube block depth and a second corrugated rib extending over the remaining third of the tube block depth. In either case, the gap is conducive to the separation of condensation water in the evaporator.

As can be seen from FIG. 4 and 5, in this example, too, the height of the heat-conducting ribs 8 and consequently the stack spacing of adjacent rectilinear flat tube sections, both within a serpentine flat tube 13 and between two adjacent serpentine flat tubes 13, correspond approximately to the height c of the return bend section 3′ which is markedly smaller than the flat tube width b. The 90° twisting, selected in this case, of the flat tube ends 6, again issuing on the same block side, does not conflict with this low stack height, since the serpentine flat tubes 13, because of their tube coil sections 12, have, overall, in each case a height in the stack direction z which is greater than the flat tube width. As mentioned, the twisting of the ends 6 through 90° at right angles makes it possible to use particularly narrow collecting ducts or header tubes forming these. FIG. 5 illustrates such a front-side header tube 7, into which the front row of flat tube ends 6 issues. Moreover, as illustrated in FIG. 5, the serpentine flat tubes 13 may be combined with the flat tube 1 of FIG. 1 and 2.

Numerous further alternatives for the two flat tube configurations shown are possible. Thus, the flat tube may have two or more return bend sections and corresponding deflections.

Moreover, the serpentine flat tube 13 of FIG. 5 may be modified to the effect that, by means of at least one further serpentine winding in one and/or other serpentine tube section, the respective flat tube end 6 comes to lie on the block side located opposite the return bend section 3. In a further embodiment, a serpentine flat tube 13 of the type in FIG. 5, but with one or more additional return bend sections 3, may be provided, in order thereby to construct, for a serpentine heat exchanger, a tube block which is at least three-part in the block depth direction. Depending on the application, the flat tube ends 6 may even be left untwisted.

In those exemplary embodiments in which the flat tube ends 6 issue on the same block side, it is possible, instead of two header tubes 7 or one common header tube into which a longitudinal partition is introduced separately during production, to use a two-chamber header tube which even at the manufacturing stage has two separate hollow chambers running longitudinally. Said header tube is manufactured from extruded profile and integrally contains two longitudinal chambers which are separate from one another and which form the collecting ducts for the respective heat exchanger. For this purpose, as in the other header tube versions, suitable circumferential slots are to be introduced into the header tube 7, the flat tube ends 6 being inserted sealingly into said slots.

Moreover, depending on the type of heat exchanger, header tubes may be used which, by means of corresponding transverse walls, contain a plurality of collecting ducts separated from one another in the vertical block direction z. As a result, the flat tubes are combined in the tube block in a plurality of groups in such a way that the tubes of a group have a parallel throughflow and the various tube groups have a serial throughflow. A thermal control medium supplied flows from an inlet-side collecting duct into the group of flat tubes issuing there and then passes at their other end into a collecting duct which functions as a deflection space and into which, in addition to this first group, a second flat tube group issues, into which the thermal control medium is then deflected. By an appropriate positioning of the transverse walls, this may be continued in any desired way, in one or both header tubes, as far as an outlet-side collecting duct, via which the thermal control medium then leaves the tube block.

The above description of various exemplary embodiments shows that, by means of the flat tubes according to the invention, highly compact pressure-resistant flat tube blocks can be produced in a single-layer type of construction or serpentine type of construction with a high heat transmission capacity. Heat exchangers produced therewith are also suitable, for example, for CO2 air conditioning systems operating at comparatively high pressure, such as come under consideration increasingly for motor vehicles. 

1. A flat tube with a return bend section (3), in which the flat tube (1) is bent around in such a way that its two planar tube sections (2 a, 2 b) adjoining it run in the longitudinal direction with opposite throughflow directions (4 a, 4 b) and with longitudinal axes (5 a, 5 b) offset relative to one another at least in the transverse direction (y), characterized in that the return bend section (3) is formed in such a way that a main bend axis (A) runs parallel to the flat tube plane and at a predeterminable angle with respect to the tube longitudinal extent, the flat tube plane being determined by the extent of the flat tube (1) in terms of length and of width.
 2. The flat tube as claimed in claim 1, further characterized in that the predeterminable angle is 90°.
 3. The flat tube as claimed in claim 1, further characterized in that the flat tube (1) is displaced by the amount of a distance (s), parallel to the tube extent, in the flat tube plane in the region of the return bend section (3).
 4. The flat tube as claimed in claim 1, characterized in that the tube sections (2 a, 2 b) merge into the return bend section (3) at a predeterminable angle (α).
 5. The flat tube as claimed in either claim 1, characterized in that the angle (a) and/or the distance (s) are/is achieved by means of at least one bending operation about at least one bend axis (B) which runs perpendicularly with respect to the flat tube plane.
 6. The flat tube as claimed in claim 5, further characterized in that the displacement of the flat tube (1) is achieved by means of two bending operations about two bend axes (B1, B2) which are carried out before or after the main bending operation about the first bend axis (A), the first bend axis (A) running in the middle of an offset region (U).
 7. The flat tube as claimed in claim 1, further characterized in that the two planar tube sections (2 a, 2 b) adjoining the return bend section (3) are arranged, with a spacing (d), perpendicularly to the stacked direction (z) in planes parallel to one another.
 8. The flat tube as claimed in claim 1, further characterized in that, in a further forming step, the return bend section (3) is formed in such a way that the two tube legs (2 a, 2 b) lie next to one another and parallel, with the spacing (d), in the same plane.
 9. The flat tube as claimed in claim 7, further characterized in that the return bend section (3) is formed symmetrically or asymmetrically.
 10. The flat tube as claimed in claim 1, characterized in that the spacing (d) in the transverse direction (y) is between 0.2 mm and 20 mm.
 11. The flat tube as claimed in one of the preceding claims claim 1, characterized in that, by means of the return bend section (3), an air-facing side of the flat tube section (2 a) becomes an air-facing side of the flat tube section (2 b) and a side of the flat tube section (2 a) facing away from the air becomes a side of the flat tube section (2 b) facing away from the air.
 12. The flat tube as claimed in claim 1, characterized in that, by means of the return bend section (3), a tube underside of the tube section (2 a) becomes the tube top side of the tube section (2 b) and a tube top side of the tube section (2 a) becomes the tube underside of the tube section (2 b).
 13. A flat tube heat exchanger for a motor vehicle air conditioning system, with a tube block (9) having one or more flat tubes (1) as claimed in claim 1, which are stacked one above the other in a stack direction (z).
 14. The flat tube heat exchanger as claimed in claim 13, characterized in that collecting ducts (7) which run along the stack direction (z) and into which the flat tubes (1) issue in each case at one end (6) are arranged laterally on the tube block (9).
 15. The flat tube heat exchanger as claimed in claim 13, characterized in that at least one of the two tube sections (2 a, 2 b) connected to one another via the return bend section (3) forms a tube serpentine (12) wound in the stack direction (z).
 16. The flat tube heat exchanger as claimed in claim 13, characterized in that the two flat tube ends (6) lie on the same or on opposite sides, and at least one of the two tube ends (6 a, 6 b) is twisted at an angle of between 0° and 90°.
 17. A gas cooler with a flat tube heat exchanger (10) as claimed in claim
 13. 18. An evaporator with a flat tube heat exchanger (10) as claimed in claim
 13. 